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What to Eat to Boost Immunity

What we eat—or don’t eat—can affect our immune system. In my video Using the Produce Aisle to Boost Immune Function I profile a study conducted to determine the effect of the consumption of brightly colored vegetables on the immune system. For the first two weeks, the subjects ate basically no fruits and veggies. Then, they drank one and a half cups of tomato juice every day for two weeks, followed by two weeks of carrot juice, and then two weeks of spinach powder. Within just two weeks of a fruit- and veggie-deficient diet, immune function plummeted. However, just one and a half cups of tomato juice a day brought subjects back from the ashes. It didn’t take five servings a day—just one tall glass of tomato juice produced results. The carrot juice alone didn’t seem to help as well, however, nor did the powder equivalent of about one serving of spinach. This tells me two things: how remarkably we can affect our immune function with simple dietary decisions and, not all veggies are alike.

When this study was repeated looking at other immune markers, the tomato versus carrot appeared more evenly matched. There is one family of vegetables, however, that we definitely don’t want to miss out on. Inflammation and leaky gut can occur all because of an absence in our diet of AHR ligands—in other words, cruciferous vegetables, including cabbage, collards, cauliflower, kale, Brussels sprouts, and broccoli.

Do people who eat healthier actually get sick less? Those who eat more fruits and vegetables appear to have a lower risk of getting an upper respiratory tract infection like the common cold, whether they’re otherwise vegetarian or not. Even just one added apple a day may help keep the doctor away. The common cold is usually so innocuous, though, so why not test against something stronger?

Researchers have also looked at more serious respiratory infections like influenza. Studying the relationship between various risk factors and influenza-related hospitalizations in the United States, they found that a 5 percent increase in the prevalence of obesity was associated with a 6 percent increase in hospitalization rate. Physical inactivity had worse outcomes, resulting in a 7 percent increase in hospitalizations. Low fruit and vegetable consumption, however, had the most impact, increasing flu-related hospitalization rates by 8 percent.

The common cold isn’t always innocuous, though. For instance, a cold during the first trimester of pregnancy is associated with a number of birth defects, including anencephaly, one of the worst, which causes a fatal malformation of the brain. More recent data suggest that the cold-related fever is the real culprit, as anti-fever drugs appear able to prevent the possible birth defects caused by the common cold.

It’s best, of course, not to get sick in the first place. One thousand women and their diets were followed before and during pregnancy. It was found that “[w]omen who consume more fruits and vegetables have a moderate reduction in risk of [upper respiratory tract infection] during pregnancy, and this benefit appears to be derived from both fruits and vegetables instead of either alone.” Whole fruits and vegetables provide a natural balance of all sorts of things that may improve our immune function in a “complementary, combined or synergistic manner that could account for the protective effect observed from high consumption of both fruits and vegetables”—or maybe that’s the only way they got enough in their diet. The women who appeared protected in this study were eating nearly nine servings of fruits and vegetables a day, compared with only five servings of fruits or four of veggies. This suggests that the arbitrary five- or six-a-day minimum may be insufficient for effective immune function.

For example, in one famous study, elderly individuals were randomized into groups that ate either five servings of fruit and veggies a day or two servings a day. The five-a-day group showed an 80 percent improved antibody response to their pneumonia vaccination compared to the two-a-day group. Even though only about 30 percent (12 out of 40 people) of the five-a-day group reached their target levels of servings, they still did six times better than the two-a-day group. But maybe eight, nine, or ten servings a day would have worked even better.

Need a reminder about what those protective Ah receptors are? See The Broccoli Receptor: Our First Line of Defense and Counteracting the Effects of Dioxins Through Diet.

What’s the best way to prepare broccoli? Check out these videos:

In late pregnancy, however, women can overdo it. See Caution: Anti-Inflammatory Foods in the Third Trimester.

What else can we do to lower our risk of upper respiratory tract infections? See:

Also be sure to check out my video, Are Happier People Actually Healthier?, which compares people’s resistance to having the common cold virus dripped into their nostrils.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:


Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous "meat defamation" trial.

66 responses to “What to Eat to Boost Immunity

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  1. I doubt if I could ever work in the (boring) numbers business. “Lies, damn lies, and statistics.”

    (I never liked Brussels sprouts either.)

    1. Figures can lie and liers can figure.

      Pregnant women should lower fever but not with Tylenol. Also no Tylenol for babies or children. Possible link with autism.

  2. I don’t know if it’s b/c I’m mostly WFPB but I can’t remember when the last time I had a cold or upper Resp. infection or flu.. Runny nose sometimes but no malaise or distress. Blow the schnoz a few times and it’s over… I’m 69 so do I qualify as an “elderly individual” ???


    1. Maybe you enjoy a wonderfully hardy constitution too Mitch as well as keeping up an active lifestyle? I have not managed to stave off shingles, vestibular neuritis (vertigo), migraine, high cholesterol or allergies unfortunately but maybe wfpb does not impact these conditions so much. For all that wfpb eating does do so easily, effortlessly, I am really thankful. The last time I had the flu/upper respiratory infection was the last time I had the flu shot. Seriously.

      1. “The last time I had the flu/upper respiratory infection was the last time I had the flu shot. Seriously.”
        – – – – – – – – – –

        Which is why I’m afraid to jinx my good luck. I never had a flu/pneumonia shot, nor have I had a bad cold or flu since the winter of 2000. Knock on wood, as they say.

      2. “Maybe you enjoy a wonderfully hardy constitution too Mitch as well as keeping up an active lifestyle?”

        I’m a old hippy mutt. Meditate, yoga, tai chi, gym and walk… Oh yeah….. I’m a 99% vegan since my early ’20’s.. Dat helps…. Just do what the smart people tell you to do to stay healthy.
        YMMV, Works for me

        1. That’s soo great Mitch! That’s a terrific combination. Good on you for being vegan all this time. I’ve been walking for years, and for the past few started meditating. There is tai chi starting here in Sept and thought it might help out with the vertigo.. it will be fun regardless, and I’m looking forward to it. All the best to you and yours.

  3. Dr G, thanks for the reminder about those very important Ah receptors (Aryl hydrocarbon). I re-watched that particular video, and continue to eat my cruciferous veggies every day. I find that the blogs on Tuesday and Thursday are very helpful.

    1. Have a look at the fodmap friendly diet, I adapted it for plant based, but importantly it shows you to do portion control to help with IBS. Monash University in Australia has the information via an app or on their website.

  4. I’m confused, are you saying that physical activity caused an increase in hospitalization…I’m assuming that you mean no physical activity, can you clarify? Thanks for all you do.

  5. When it comes to boosting immunity remember to add ground cardamon and black pepper your the bowl of blueberries.

    This is as per Dr. Greger’s video.

      1. Sokay. :-)

        Since you mentioned blueberries: Remember when I told you Wyman’s would be sending me some coupons? Well, in today’s mail I see they sent me two (2) free coupons for any one bag of Wyman’s of Maine Frozen Fruit. I’ll probably wait until the bigger bag goes on sale though. Says here the “maximum value is $11.99. No expiration date either.

        How ’bout them “apples”? :-)

      1. With Cardamom, it wasn’t about ORAC values. Cardamom increased how well the Natural Killer cells worked. Blueberries doubled how many there were. Cardamom and pepper increased how well the Natural Killer cells killed Cancer.

  6. my wish for the doc–how can we get these benefits without stuffing ourselves to the gills with vegetable matter every day. 9 servings a day. seems ridiculous and what damage caused by overeating?

    1. @frank
      1 serving is 1/2 a cup. If you eat 2 servings for each of your 3 meals, you have already gotten 6 servings! Then add a smoothie consisting of at minimum 1 and 1/2 cup of vegetables + 1 and 1/2 cup of fruits and you have your 9 servings!

  7. “in other words, cruciferous vegetables, including cabbage, collards, cauliflower, kale, Brussels sprouts, and broccoli. LOL. You just named all my favorite vegetables. As a person allergic to mushrooms, onions, peppers, and all squashes, to me this is the cornucopia of the vegetable world. And to the guy commenting about 9 servings a day, he obviously hasn’t tried. As any good WFPB eater can tell you, our stomachs revel in all those veggies, and our bowels do to.

  8. I am so over with this “x servings” per day nonsense. What is a serving? No one I know has an answer. How about “x ounces/body weight” or something akin? Related, does a 200 pound person need more servings than a 120 pound person? There is definitely a lack of precision in the Doc’s message.

    1. I agree. By now, you probably know how your body operates in order to stay healthy. Getting all angst-y over exact measurements of this or that is too much “sweating the small stuff.” IMHO.

      1. I agree, YR & Barbie. If someone’s getting all angst-y about servings, they’re probably over thinking it. Serving sizes stated in the NF app have been a good guide for me in the past when I wasn’t sure or had a question.

        Just relax & have another fruit or vegetable ;-)

        1. The Daily Dozen app is awesome. I don’t really need to use it anymore, but I still love to tick off the boxes.

          I thought I was doing really well until I started using the app. It made me realize that needed to eat more fruit & legumes. Now it helps keep me on track. If I have a choice between eating something that fits the app criteria & something that doesn’t, I go for what fits.

    2. Paul,

      Don’t let the number of servings be daunting to you. He is just pointing out that 5-servings of produce wasn’t enough for 70 percent of the people and he is estimating how many servings might help a higher portion of the people.

      The scientists used “servings” as the measure they chose to use.

      So does the government.

      I agree with you that body weight does affect serving size.

      The average person isn’t measuring and weighing their food unless they are on a program, which gets them to do it. Most people just put the food on the plate in front of them and eyeball it. A half cup is easier to eyeball than an ounce is probably why they chose it.

      With WFPB, many of the people are just eating until they are full, and don’t keep track of anything at all.

      If you aren’t getting sick, then you might already be okay. If you are getting sick, then add as much as you are comfortable with.

      1. Thanks for the response.  I am a scientist too (not in this field).  If I used a nonsense term like “servings” I would not pass muster with my peers and my papers would be rejected.  I agree “cups” is easier to visualize, but how much is a cup?  A loose cup and a packed cup carry much different quantities.

        1. Paul, If you want accuracy, just look up the portion size in grams. Get a food scale, and you will see how much that is. Only have to do that once or twice, then you’ll know how much is a serving of that particular food.
          If you eat mainly fruits and vegetables, which includes beans and other legumes, 9 servings isn’t really a lot.
          The idea is to cut the ‘fluff’ out of your diet and eat the good stuff.

        2. Paul,

          You are absolutely correct and I love that there are scientists in the mix.

          If you came to my house, we would have long conversations over that. I have a few friends who are scientists and science deals in that type of precision and I would want you to have a precise understanding of things.

          I watched The Impossible burger scientists and they blew me away, even if I never eat their product, because every nuance mattered.

          Then, I watched NASA’s food scientist team:

          I also have a lot of art major friends and they would laugh at the concept of measuring food at all.

          Here would be what my artistic friends would do with the food:

          As for me, I think that, as long as you pack your own, it is not hard to measure an ounce or a cup or a gram, it is translating between the various systems, which is confusing.

          1. I am laughing because I just realized my own influences didn’t even use measuring cups or scales of any kind.

            My grandmother on one side of the family was not good at speaking English, but what I marveled about her was that she cooked without using measuring cups or measuring spoons or mixing bowls. She would crack eggs onto the counter and do the whole process with her hands.

            My grandmother on the other side was strongly from a poverty and war rationing mentality. She had recipes, but she had concepts, “If you don’t have eggs, do this….” “If you don’t have sugar, do that…” Her recipes were a pinch of this, a pinch of that, with 90% exceptions.

            When she was young, the family often didn’t have food, because her father was a seasonal worker. Then, he was a barber, where people would show up without money and paid him in alcohol.

            Those two women would never be able to answer your questions, but they would both invite you to their table.

  9. I think I like the tomato juice part the most.

    I like cruciferous vegetables, but somehow find them harder to eat every day all year round.

    I did really well for 6 months, but I start not wanting them and need to take a month off and go back on.

    Tomato juice, I can do every day. There is something comforting about it.

    Forgive me, but even though I genuinely do like cruciferous veggies, they are almost “un-comfort” food. I started my superfood wraps again, but I left out the raw broccoli and cauliflower. I will take this as if it was a letter from Dad saying, “Have you stopped eating your broccoli again?”

    Can I buy jars of tomato juice as long as I find a brand without a lot of sodium or is it more like juicing where you lose the enzymes if it is pasteurized?

    1. I could call them discomfort food sometimes, but more often I think they are not “comfort food”, at least raw and when they are cooked, I feel unsure if they have lost their super powers.

      1. Studies have shown that highest nutrients in broccoli are after steaming for no more than 4 minutes. Set a timer.
        I make a lot at once, then just add cold to salads, or eat as a snack.

    2. Broccoli is one of my faves…along with kale and collard greens. But broccoli raw? No way; I always lightly steam them.

      Never got into the smoothie habit, as much as I’m sure they’re really healthy and delicious. One of the main reasons is because I usually take city buses (or walk) to get my greens, etc. To make just one smoothie would require that I trek back more bags than I care to deal with in just one bus trip.!page=post&id=57A34CA2-9E10-836C-4DF0-AB840F8FD33D

      1. Yes, public transportation limits the load.

        You saying that sentence is perfectly timed.

        I think I am going to make some smoothies again.

        The thing is, I wanted to get some tomato juice, but halfway to the store, I started thinking that I would like some dairy free ice cream and that knocked the thoughts of tomato juice out of my brain.

        I am sitting in the car just before going in to the grocery store for.,,,

        And I have to fill in the blanks before walking in.

        Tomato juice and something to approximate ice cream without all the sugar is the thought that is wrestling to knock dairy free ice cream off the invisible list.

        1. This is the first:

          Flashing warning sign for me.

          I craved ice cream.

          Wondering why my brain isn’t getting a memo to crave smoothies? Almost the same thing.

          I am adding the fruit and vegetables back in just in the nick of time.

          I am not craving real ice cream or real cheese or anything animal.

          It has to be sugar.

  10. My favorite plant food for immune function are the mushrooms, shitake, maitake, etc. Many studies attest to the benefits of various type of mushrooms.
    There is an Asian food store here with quite a selection. Fun to try different kinds.

    1. Marylin,

      I have ended up doing mushroom supplements for my dog who has Cancer and I am respecting mushrooms more than ever.

      He has such an aggressive Cancer and it had already spread through his system, but when I read that they had kept dogs alive for a year with Turkey Tail Mushrooms, that is amazing to me.

      Many of the mushrooms have these amazing health benefits.

      Are you cooking them?

      I bought the supplements, because I am not good at remembering to cook them and find myself eating them raw in my salads or wraps.

      Bummer that they have anything at all potentially toxic.

    2. Marilyn, I bought some shitake mushrooms from a within-walking-distance supermarket the other day. They sell their mushrooms at a good price there so I often pick up that kind.

      But their stems are tough little suckers — hard as rocks! Do you just cut them in smaller pieces and steam as usual? (I never eat mushrooms raw…do you?)

      1. YR, I cut the stems off, then sauté the tops in veggie broth.
        You can save the stem pieces to make your own broth. Just strain them out after. They add that unami flavor to soups. I love barley soup. Miso also.

  11. What’s a serving of fruits and veggies? 9 servings a day – is it 9 plates/bowls of salad? If I eat in total 3 plates of food a day, how can I eat so much veggies?… In my country nobody ever measured food by some “servings”, only by grams or portions (which is a full plate of food). When writing such articles why not to detail what’s a serving?

    1. Ours isn’t a whole plate of food.

      If you look on a site like Amazon, they show portion plates where you could see visually what a serving size is.

      Weight Watchers and Sparkpeople are likely to have videos on it.

      They also sell bowls in 1/2 cup, 1 cup, 2 cup, on-line.

      When I cook chili or beans and rice, I often divide the food into 2 cup bowls. Why not one-cup bowls? Because two cups is generally filling and one cup is generally not filling and if I am not full, I suddenly think about ice cream.

      1. I thought I could help by finding a gram to 1/2 cup conversion calculator, which I did find, but I clicked on and 1/2 cup weighs differently for every ingredient.

        I guess that makes sense, but it means that the grams are constantly changing fruit to fruit and vegetable to vegetable, even if they are all 1/2 cup.

        That makes sense if you pick up grapes and then pick up a grapefruit.

        Anyway, it turns out that it would be hard to give you what you want.

    2. In many countries, food labels are required by law to show the number of servings in the packet, box etc

      In the USA, the government decides how big is a serving for many fruits and vegetables. The government information about servings is also shown in ounces and grammes eg

      They do the same thing in Europe eg,-Diet-and-Health/Eating-In/Serving-sizes.aspx

      What is your country?

  12. As a senior, age 75, friends told me to limit shade vegetables to avoid arthritis. I have given up tomatoes, eggplant (and I love tomato juice). Can U discuss this?

    1. Alice, some people do have a reaction to nightshades. But anyone can have a reaction to any food. There is absolutely no reason to stop eating these nutritious foods if they don’t bother you.
      The idea that they cause arthritis in everyone is wrong.

  13. I’m wondering, when talking about the benefits above of drinking tomato juice or carrot juice daily – Did they drink juice pressed from fresh tomatoes and carrots, or did they use a commercially sold product? I know the benefits of juicing fresh vegetables, but I cannot afford a juicer, and also cannot afford to buy the tremendous amounts of produce it takes to juice daily. Are there any commercial juices that are less processed and less ‘bad’ for you than most of them?

  14. Maureen,

    The products appear to be commercial mixes that were standardized to contain (37.0 mg/day lycopene) for the tomato juice and for the carrot juice (27.1 mg/day beta-carotene and 13.1 mg/day alpha-carotene).

    On the second question let’s talk about what you mean by juicing. There are two primary approaches in juicing. One is to separate the liquid vs solids (think Omega or Champion) vs the other approach being a blending of sorts which uses high speed blades to emulsify the mix (Vitamix) retaining the complete item.
    The best approach would be….whole product.

    Dr. Alan Kadish moderator for Dr. Greger

  15. I have a blood condition called Hereditary Spheryistosis, it’s been mild my entire life (34 years old) except that my spleen is enlarged due to constant killing off and regenerating of blood cells.
    Recently within the last year my spleen has gotten so large I can feel it often, have some mild discomfort when eating, and my stomach sticks out in the area. It’s currently 21-22cms. I am having elective splenectomy due to its size on October 25th. My condition isn’t critical, my counts are satisfactory considering my history and my wfpb diet. It’s mainly just the fear of it getting larger (open surgery after 23cm vs laparoscopically at 21-22cm) that has me doing it now, timing of family available to help with my kids etc etc.

    My really concern and question is life post spleen- Asplenic.

    What do you think, recommend and suggest to me?

    1. Hello Jenn,

      I’m very sorry to hear about your health struggles and I sincerely hope the surgery helps. As for your question about post-surgery. That’s something that you’ll need to discuss with your doctor and surgeon. Are you able to book a consultation with the surgeon prior to the surgery?

      I hope this helps,
      Dr. Matt

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